When functioning properly, the human heart maintains its own intrinsic rhythm, and is capable of pumping adequate blood throughout the body's circulatory system. However, some people have irregular cardiac rhythms, referred to as cardiac arrhythmias. Such arrhythmias result in diminished blood circulation. One mode of treating cardiac arrhythmias uses drug therapy. Anti-arrhythmic drugs are often effective at restoring normal heart rhythms. However, drug therapy is not always effective for treating arrhythmias of certain patients. For such patients, an alternative mode of treatment is needed. One such alternative mode of treatment includes the use of a cardiac rhythm management system. Portions of such systems are often implanted in the patient and deliver therapy to the heart.
Cardiac rhythm management systems include, among other things, pacemakers, also referred to as pacers. Pacers deliver timed sequences of low energy electrical stimuli, called pace pulses, to the heart, such as via an intravascular leadwire or catheter (referred to as a “lead”) having one or more electrodes disposed in or about the heart. Heart contractions are initiated in response to such pace pulses (this is referred to as “capturing” the heart). By properly timing the delivery of pace pulses, the heart can be induced to contract in proper rhythm, greatly improving its efficiency as a pump. Pacers are often used to treat patients with bradyarrhythmias, that is, hearts that beat too slowly, or irregularly.
Cardiac rhythm management systems also include cardioverters or defibrillators that are capable of delivering higher energy electrical stimuli to the heart. Defibrillators are often used to treat patients with tachyarrhythmias, that is, hearts that beat too quickly. Such too-fast heart rhythms also cause diminished blood circulation because the heart isn't allowed sufficient time to fill with blood before contracting to expel the blood. Such pumping by the heart is inefficient. A defibrillator is capable of delivering an high energy electrical stimulus that is sometimes referred to as a defibrillation countershock. The countershock interrupts the tachyarrhythmia, allowing the heart to reestablish a normal rhythm for the efficient pumping of blood. In addition to pacers, cardiac rhythm management systems also include, among other things, pacer/defibrillators that combine the functions of pacers and defibrillators, drug delivery devices, and any other implantable or external systems or devices for diagnosing or treating cardiac arrhythmias.
One problem faced by cardiac rhythm management systems is in providing diagnostic data to the physician or other caregiver to assist in determining the efficacy of therapy being delivered by an implanted cardiac rhythm management device. Such diagnostic data is typically displayed on the screen display and/or recorded on a strip chart recording, each of which is provided by a user interface portion of the cardiac rhythm management system, such as a programmer. The programmer is external to the patient and therefore is remote from the implanted cardiac rhythm management device.
Displayed diagnostic data may include, among other things, one or more representations of intrinsic electrical signals produced by the heart, which are referred to as “cardiac signals” or “heart signals.” Such cardiac signals include, among other things, electrical depolarizations associated with heart chamber contractions and electrical repolarizations associated with heart chamber expansions. Decisions regarding the delivery and/or withholding of cardiac rhythm management therapy are often based at least in part on information included within such cardiac signals. Moreover, cardiac signals may include information indicating the efficacy of therapy that has already been delivered. The cardiac signals obtained by the programmer may include both surface electrocardiogram (ECG) signals, obtained by external ECG skin electrodes yielding ECG signals directly coupled to the programmer via a cable, and electrograms, obtained by implanted electrodes yielding electrogram signals telemetered from the implanted device to the programmer in a real-time data stream.
The real-time data stream from the implanted cardiac rhythm management device to the external programmer may include other data that provides useful diagnostic information to the caregiver. For example, event markers may be displayed on the screen display and/or strip chart recording of the programmer together with the surface ECG and/or electrogram signals. Such event markers annotate the particular times at which certain events associated with the implanted device occur. In one example, representations of such event markers appear on the strip chart recording as upward pointing arrows to indicate the time of occurrence of the marked event, along with a text annotation identifying the type of marker and/or providing other useful information such as the time between successive markers associated with the same chamber of the heart. In this example, “AS” indicates that the associated event marker corresponds to an atrial sense by the implanted device, “AP” indicates that the associated event marker corresponds to an atrial pace by the implanted device. Similarly, “VS” indicates that the associated event marker corresponds to a ventricular sense by the implanted device, “VP” indicates that the associated event marker corresponds to a ventricular pace by the implanted device. Many other marker types exist and are identified by other text annotations. Markers may include additional information. In one example, a corresponding numeral indicates the time interval in milliseconds since the previous event marker in the same chamber. All this information is useful to the caregiver in diagnosing whether the implanted device is providing appropriate cardiac rhythm management therapy.
Implanted cardiac rhythm management devices are designed for low power consumption. This extends the useful life of the battery-powered implanted device before explantation and replacement is required. As a result of power or other limitations, implanted cardiac rhythm management devices typically provide slow telemetry communication of data to the external programmer. In one example, each “frame” of transmitted data includes signal information from first and second (e.g, atrial and ventricular) electrograms as well as marker information from first and second heart chambers (e.g., atrial and ventricular markers).
In this example, each event marker typically includes enough data so that it takes several frames of data to completely transmit the information associated with a single event marker. Thus, a time delay exists between the occurrence of the event and the receipt of the event marker by the external programmer. However, to be useful to the caregiver, the event markers must be displayed on the screen and/or strip chart recording at the proper time relative to the representations of surface ECG and electrogram signals that are being received and displayed without such delay. The surface ECG and electrogram signals may be buffered and delayed by a fixed delay, which is approximately equal to the transmission time between occurrence of the event and receipt of the event marker by the programmer. This allows the external programmer to “align” the event markers in time to the surface ECG and electrogram signals being displayed on the screen and/or strip chart recording. However, if different types of markers are used, and these markers have different transmission times, then buffering the cardiac signals by a fixed delay will not provide proper alignment between the displayed representations of the event markers and cardiac signals, particularly if events occur during the transmission time of a previous event marker associated with the same heart chamber. Thus, there is a need for improved techniques for accurately displaying such diagnostic information for the caregiver.